On the deceleration behaviour of black holes

On the deceleration behaviour of black holes
Kicking about in space: Researchers have modelled the collision of black holes (1). Thereby it was shown that the newly created black hole is initially deformed (2). To smooth this asymmetry and achieve the energetically more favourable spherical shape, more momentum is radiated upwards: this "anti-kick" decelerates the black hole slightly - it now continues to move with reduced speed (3). Image: MPI for Gravitational Physics

(PhysOrg.com) -- Researchers use the concept of "anti-kick" to explain why the speed suddenly decreases after the collision of such exotic objects.

Kicking is not only associated with football: if two approach each other so closely as to collide and merge, the resulting black hole recoils and then races through the universe at a speed of up to several thousand kilometres per second. Sometimes, however, it experiences a sudden decrease in speed - a behaviour for which there was no convincing explanation. Scientists from the Max Planck Institute for Gravitational Physics have now found a solution to the puzzle: there is a type of recoil in the opposite direction that reduces the speed of the whole system. In this "anti-kick" the black hole emits to reach its energetically optimum shape: a sphere. (, June 3, 2010)

Viewed from the outside, a black hole is not a tangible object but a region in space that draws in matter from its surroundings with great force. The boundary which separates this region from the rest of space is called the horizon. In the simplest case, the horizon is perfectly spherical and floating in space. Anything crossing the horizon from the outside is unable to leave again. Not even light can escape this gravitational trap - hence its name. Black holes are considered to be important components of models which astrophysicists use to explain or the interior of .

Luciano Rezzolla, head of the ‘Numerical Relativity’ research group at the Max Planck Institute for ( Institute, AEI), and his colleagues Rodrigo Macedo and José Luis Jaramillo started by examining a simple system. In this model, a smaller and a large black hole move linearly towards each other and collide head-on. The smaller black hole moves faster, has a high downward momentum and emits strong gravitational waves downwards. Since every action also produces a reaction, the overall system moves upwards - this is the "kick" (left part of Fig.).

The black hole produced by the merger is initially not spherical, but deformed and has a type of "bump" at the top (central part of Fig.). To smooth this asymmetry and achieve the energetically more favourable spherical shape, more momentum is radiated upwards by means of gravitational waves: this "anti-kick" therefore decelerates the resulting black hole. It still moves upwards, albeit at a reduced speed (right part of Fig.).

Inspiralling black holes video simulation.

"This simple model takes us a lot further towards understanding black hole collisions. In our publication we provide an intuitive explanation for a process whose mathematical details are terribly complex," says Luciano Rezzolla. "It is important in physics to understand complex phenomena and provide clear explanations. This is the significance of the research results of Rezzolla and his team," adds Bernard F. Schutz, Director of the Astrophysical Relativity Division.

The generation of a large kick in the merger of black holes has a direct impact in astrophysics: Depending on its size, in fact, it determines the number of galaxies containing supermassive black holes at their centres since black holes can even be kicked out of their galaxies. The explanation of the anti-kick suggests a methodology to probe the physics around a black hole by monitoring the geometry near its horizon. This approach may help understanding some fundamental aspects of black-hole physics.


Explore further

How do supermassive black holes get so big?

More information: Luciano Rezzolla, Rodrigo P. Macedo and José Luis Jaramillo, 'Understanding the "anti kick" in the merger of binary black holes', Physical Review Letters, June 3, 2010, prl.aps.org/abstract/PRL/v104/i22/e221101
Citation: On the deceleration behaviour of black holes (2010, June 4) retrieved 21 August 2019 from https://phys.org/news/2010-06-deceleration-behaviour-black-holes.html
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Jun 04, 2010
"speed decreases" in what frame of reference?

Jun 04, 2010
"speed decreases" in what frame of reference?


Every frame of reference except the one within the black hole. In that frame of reference, the entire universe is moving and then reduces speed.

Jun 04, 2010
There's an intrinsic "upward" and "downward" and differentiated relative momentums in this two-body collision? Really?

Obviously, they must be using an external frame of reference. So therefore, you'd expect the overall momentum to be the sum of the two parts. It isn't clear here how this is any different.

Are they essentially saying that some of the momentum is lost through friction with spacetime, via the gravity waves?

Has anyoone yet detected any such gravity waves?


Jun 04, 2010
Talk about a load of crap! Sorry to all you brilliant guys out there who spend eons working all this out, but has this behaviour been observed anywhere? Oh and the guys with the "boson condensate".....really?
I guess when you have something that is unknown and unknowable, you can call it what you want, but for those of us who live in the real universe, this is nothing but a load of what Schroedinger's cat left in the box.

Jun 04, 2010
I've never heard of the "event horizon" called just the "horizon". Is this what the youngsters these days are calling it to be "cool" or is this author not familiar with the term "event horizon"?

Jun 05, 2010
Just curious... Is anyone reading Nisaj's posts? I'm not making a point... It's just that there's SO MUCH posted by this one author on this story and I can't make heads or tails out of what he (or she) is trying to say.

Jun 05, 2010
Nisaj/Alizee etc are well known, and best avoided.

Jun 05, 2010
Just curious... Is anyone reading Nisaj's posts? I'm not making a point... It's just that there's SO MUCH posted by this one author on this story and I can't make heads or tails out of what he (or she) is trying to say.
I have this nagging fear that Nisaj/Alizee is an alien from another world, sent here to tell us something very important, but s/he can't dumb it down enough for us!

Aack!

Jun 05, 2010
Two objects falling into each other are going to end up moving slower once they "combine" they are heavyer...

Jun 06, 2010
Two objects falling into each other are going to end up moving slower once they "combine" they are heavyer...

The gravitational center will continue to move at the same speed and at the same rate and in the same direction before and after the collision. The two bodies will end up as one going at that vector.

This article explains an exception to this where speed is lost to gravitational waves. I'm a little skeptical of that because in which inertial reference frame do we consider it "slowed"? If it slows in one inertial frame, it'll speed up in another. "Speed" is relative. Perhaps better wording would be, "the whole system's center of gravity vector changes".

Jun 06, 2010
The gravitational center will continue to move at the same speed and at the same rate and in the same direction before and after the collision. The two bodies will end up as one going at that vector.

This article explains an exception to this where speed is lost to gravitational waves. I'm a little skeptical of that because in which inertial reference frame do we consider it "slowed"? If it slows in one inertial frame, it'll speed up in another. "Speed" is relative. Perhaps better wording would be, "the whole system's center of gravity vector changes".
I'm having trouble reconciling this apparent acceleration paradox. Technically (If I'm understanding the article correctly) the acceleration occurs in the direction of the gravity waves. This would mean that by discarding energy, the system increases its momentum/KE in the same direction. This seems to be a conservation violation.

continued...

Jun 06, 2010
The only way this works for me at all is if there is some sort of connectivity/friction with the surrounding spacetime (the black hole essentially being a spacetime object itself). However, if this were true it seems to me it should always be apparent in some regard, thereby implying that spacetime is absolute.

If that's true, then embedded objects (such as ourselves) could never perceive these gravity waves, and the black hole could never accelerate via gravity waves relative to us anyway!

Aack!

Anyone got any aspirin?

Jun 06, 2010
I think what the article is saying is that there is an asymmetry in the emission of gravity waves, and of course every action has an equal and opposite reaction.

Hmmmm.....I'm visualizing this like when a drop of water falls into a puddle and the waves rapidly move in rings both away from the collision point and back towards the center. With water the waves are of sufficient intensity to kick back some microdroplets at high speed away from the collision.

I wonder if it is possible in some black hole collisions that the gravity waves can have sufficient focus and intensity to "squirt" tiny black holes at extremely high speed from the point of impact.

Jun 06, 2010
As nobody has any practical experience with the dynamics of strong gravitational fields, intuition and analogies are of no help to begin with. The first steps must be done numerically and only thereafter we may strain our phantasies to visualize the results. Mathematics rules, imagination follows.
That's not how Einstein did it.

Jun 06, 2010
I don't see why my comment on a heavyer object will move slower was so stupid?

Jun 06, 2010
I don't see why my comment on a heavyer object will move slower was so stupid?

I don't think it was stupid. I think it (the results of the research in this article more than your comment) ignores simple relativity. All objects in free space are standing still, moving right, up, down, left, towards a viewer, away from a viewer, fast, slow, all at the same time. It's all a matter of where the viewer is relative to the object. Imagine Alice and Bob whizzing past you quickly at exactly the same speed in exactly the same direction. From /their/ perspective, /they're/ sitting still and /you're/ moving. Imagine a 3rd viewer whizzing past you in the same direction, but 1/2 the speed. Now /he/ disagrees with all of your measurements. Who's right? You all are. There's no absolute position, speed, or direction in free space, so you can't say, universally, that either black hole "slows down". That has no meaning. To some viewers, it speeds up. To others, it was still, then moved.

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